Compounds effective in inducing cell differentiation and process for preparing same

ABSTRACT

This invention provides novel side chain homologs of 1α,25-dihydroxyvitamin D 3  which exhibit enhanced and highly selective activity in inducing differentiation of malignant cells. It also provides a general method of synthesis applicable to the preparation of a variety of vitamin D side chain analogs, and a method of treatment of neoplastic diseases which takes advantage of the selective differentiation activity of the new vitamin D homologs.

This invention was made in the course of work supported by grants orawards from the Department of Health and Human Services. The Governmenthas certain rights in this invention.

This is a continuation of copending application Ser. No. 07/187,680filed Apr. 29, 1988 now U.S. Pat. No. 4,927,815.

This invention relates to novel vitamin D compounds and to a generalprocess for their preparation. More specifically, the invention concerns1α-hydroxyvitamin D side chain homologs, which are specifically andunexpectedly active in differentiating malignant cells, and to methodsof use of the new compounds for the treatment of neoplastic diseases,including specifically leukemoid diseases.

BACKGROUND

The compounds of the vitamin D series are well known as agents essentialfor controlling calcium homeostasis in the animal or human. It is alsoknown that it is not vitamin D itself, but metabolites generated from itin the animal or human body, that are effective in regulating calciummetabolism. In this context, the most important vitamin D metabolite is1α,25-dihydroxyvitamin D₃ (1,25-(OH)₂ D₃). This compound, as well ascertain structural analogs, such as 1α-hydroxyvitamin D₃ (1α-OH-D₃),1α-hydroxyvitamin D₂ (1α-OH-D₂), or certain fluorine substituted1,25-(OH)₂ D₃ derivatives, are highly potent in stimulating intestinalcalcium absorption, as well as the resorption of calcium from bone (bonemobilization). As a consequence, these compounds are now used, or havebeen proposed for use, as pharmaceutical agents for the treatment of avariety of calcium metabolism disorders, such as renal osteodystrophy,hypoparathyroidism, vitamin D-resistant rickets, or osteoporosis.

More recent research has established that 1,25-(OH)₂ D₃, in addition toits role in regulating calcium homeostasis in vivo, also expresses otherbiological functions. Specifically, it has been shown that 1,25-(OH)₂D₃, and compounds closely related to it (e.g. 1α-OH-D₃, or fluoroanalogs of 1,25-(OH)₂ D₃) are highly potent in inducing celldifferentiation. Most importantly, it has been found that 1,25-(OH)₂ D₃will inhibit the proliferation of malignant cells (specifically,leukemia cells) and bring about their differentiation, in culture, tonormal monocytes [Abe, et al., Proc. Natl. Acad. Sci. U.S.A. 78, 4990(1981); Honma et al., ibid, 80, 201 (1983)]. Because of this remarkableactivity, 1,25-(OH)₂ D₃ and related compounds have been proposed asanticancer, specifically antileukemic, agents (Suda et al., U.S. Pat.No. 4,391,802). However, even though these compounds are, indeed, highlyeffective in differentiating malignant cells in culture, their practicaluse in differentiation therapy as anticancer agents is severely limitedbecause of their equally high potency as agents affecting calciummetabolism. At the levels required in vivo for effective use asantileukemic agents, these same compounds can induce markedly elevatedand potentially dangerous blood calcium levels by virtue of theirinherent calcemic activity. This calcemic activity precludes, orseverely limits, the use of these known vitamin D compounds in thetreatment of malignancies, and indicates a need for compounds withgreater specificity and selectivity of action.

In this disclosure the terms `calcemic activity` or `calcemic action`are intended as a short-hand designation of the well-known ability ofvitamin D compounds to raise blood calcium levels by virtue of theirstimulation of intestinal calcium absorption (Ca transport) and ofcalcium resorption from bone (bone mobilization). The term`differentiation activity` refers to the more recently discoveredactivity of certain vitamin D compounds in arresting the proliferationof malignant cells and inducing their differentiation to normal cells.

Previous work has led to the preparation of several compounds withenhanced differentiation activity. Thus, U.S. Pat. No. 4,717,721, aswell as other publications [Ostrem & DeLuca, Steroids, 49 73-102 (1988);Ostrem et al., J. Biol. Chem. 262, 14164 (1987)] disclose that1,25-(OH)₂ D analogs in which the side chain is elongated by one carbon,exhibit a differentiation activity for leukemia cells about ten timesgreater than 1,25-(OH)₂ D₃ itself. However, such compounds still areapproximately as potent as 1,25-(OH)₂ D₃ in stimulating calciumabsorption and elevating serum calcium levels, and thus do not overcomethe problem of the undesired potent `calcemic action` discussed above.Thus, although such compounds show an improved differentiation/calcemicactivity ratio, they are not selective in that their calcemic potency isas high as that of the parent compound (1,25-(OH)₂ D₃). Other vitaminD-related compounds, which are said to exhibit preferentialdifferentiation activity, have been reported [Ostrem et al., supra;Kubodera et al. Chem. Pharm. Bull. 34, 2286-89 (1986); Ikekawa et al.Chem. Pharm. Bull. 35, 4362 (1987)], but these are structurally distinctand different from the compounds of the present invention.

SUMMARY OF THE INVENTION

The present invention provides a novel class of compounds exhibiting adesired, and highly advantageous, pattern of biological activity. Thesecompounds are characterized by a high potency (compared to that of1,25-(OH)₂ D₃) in inducing the differentiation of malignant cells, whileexhibiting much lower activity than 1,25-(OH)₂ D₃ in their effect oncalcium metabolism. Hence, these compounds are highly specificdifferentiation agents, and their activity pattern would allow their usein differentiation therapies of malignancies. Their very highdifferentiation activity combined with their markedly reduced orabolished action on calcium metabolism allows the in vivo administrationof these compounds for the treatment of malignancies without inducingexcessively elevated blood calcium levels. Because of suchcharacteristics, they would be the preferred therapeutic agents for suchpurposes.

Structurally, the key feature of the compounds having these desirablebiological attributes is that they are side chain homologs of 1,25-(OH)₂D₃ in which the side chain is elongated by insertion of two or threemethylene units into the chain. Thus, the compounds of this type arecharacterized by the following general structure: ##STR1## where X, Yand Z, which may be the same or different, represent hydrogen or ahydroxy-protecting group, and where n is an integer having the values 3or 4.

24-Dihomo-1,25-(OH)₂ D₃ and its hydroxy-protected derivatives, that isthe compounds having the structure shown above, where n is 3 and24-trihomo-1,25-(OH)₂ D₃ and its hydroxy-protected derivatives, i.e. thecompounds as shown above where n=4 are preferred examples of thestructurally defined compounds above.

The compounds of this invention are related to the 24-homo-vitamin Dcompound of U.S. Pat. No. 4,717,721. However, the compounds of thisinvention can be distinguished by both structural and biologicalcharacteristics. Structurally, the compounds are novel vitamin Dhomologs with the key characteristic that the side chain is extended byinsertion of two or three methylene units into the carbon chain, andbiologically, the compounds are highly selective cell differentiationagents with a potency at least similar to or greater than that of1,25-(OH)₂ D₃ in inhibiting the proliferation and inducing thedifferentiation of malignant cells, while exhibiting no, or greatlyreduced, calcemic activity.

The present invention, therefore, provides novel compounds specificallyuseful for promoting the differentiation of malignant cells, and for thetreatment of neoplastic diseases.

This invention also provides a new method for the preparation of vitaminD compounds. This method can be used for the synthesis of the newcompounds shown above, as well as for the synthesis of known vitamin Dmetabolites or other homologs, or other side-chain modified vitamin D₃derivatives.

The basic concept of this synthesis is the elaboration of the desiredvitamin D compound by coupling the appropriate and separatelysynthesized side chain unit to a preformed vitamin D nucleus bearing adisplaceable group at carbon-22. The required side chain unit isprepared as a phenylsulfone derivative, and the required vitamin Dnucleus, when 1α-hydroxyvitamin D-type compounds are the syntheticobjective, is a secosterol derivative of the following structure:##STR2## wherein X and Y represent hydrogen or hydroxy-protectinggroups, and where L is a leaving group such as halide, or a tosyloxy ormesyloxy or similar displaceable function. A preferred derivative ofthis type is the hydroxy-protected vitamin D-22-tosyloxy compound. Thenecessary phenylsulfone compounds used for the construction of thedesired side chain have the following structure:

    PhSO.sub.2 --CH.sub.2 --R

where Ph represents a phenyl or alkyl substituted phenyl group, andwhere R is selected from the group consisting of alkyl, and hydroxy- andfluoro-substituted alkyl, where the hydroxy groups are preferablyderivatized by hydroxy-protecting groups.

The coupling between the above described vitamin D-22-tosylate, and theappropriate phenylsulfone unit, is based on established principles ofsulfone chemistry [e.g. see P. D. Magnus, Tetrahedron 33, 2019 (1977);Trost et al., Tetrahedron Letters, 3477 (1976)]. The reaction, mediatedby a strong base, yields an adduct, which may be represented by thefollowing structure: ##STR3## wherein R, X and Y represent groupings asdefined above. Intermediary adducts of the type shown above, which arenew compounds, may then be reduced in a medium containing a metalamalgam (e.g. sodium amalgam, aluminum amalgam) to produce the desiredvitamin D compound of the general structure: ##STR4## Reductivedesulfonation can also be achieved by other means, such as dissolvingmetal reductions, using, for example, metal/alkylamine or metal/NH₃mixtures. Hydroxy-protecting groups can then be removed by standardmethods known in the art, so as to produce the corresponding freehydroxy compounds.

The above described vitamin D-22-tosylate starting material for the sidechain coupling process of this invention is a known substance which canbe prepared by known methods [e.g. Andrews et al., J. Org. Chem. 51,4819 (1986)]. It can also be prepared by hydride reduction, andsubsequent tosylation of a vitamin D-22-ester [see also Kutner et al.,Tet. Letters 28, 6129-6132 (1987)] having the following structure:##STR5## wherein X and Y are hydrogen or hydroxy-protecting groups, andwhere A represents an alkyl or aryl group. The preparation and use ofthese novel esters as part of the present synthesis of vitamin Dcompounds is another aspect of this invention.

It is evident that the above-described process, using the vitaminD-22-tosylate as the vitamin nucleus and a suitable phenylsulfone as theside chain residue, can be used for the preparation of many1α-hydroxyvitamin D side chain analogs, depending upon the alkyl orhydroxy-alkyl radical R selected. Preferred phenylalkyl sulfone units,are those compounds in which R is an alkyl or a hydroxy-alkyl radical ofthe following general structures: ##STR6## where U is selected from thegroup consisting of hydrogen, hydroxy, protected-hydroxy, or ahydrocarbon radical of 1 to 4 carbons, and where p, m, and n areintegers having, independently, the values 1 to 5, and where Z ishydrogen or a hydroxy-protecting group. Especially preferred for thepreparation of novel compounds having the desired biological activity(i.e. high differentiation activity and no or low calcemic activity) arethe sulfone units shown above in which n has the values 3 or 4.

As used in the description and in the claims, the termhydroxy-protecting group signifies any group commonly used for thetemporary protection of hydroxy functions, such as for example, acyl,alkylsilyl, and alkoxyalkyl groups, and a protected hydroxy group is ahydroxy function derivatized by such a protecting group. The term `acyl`signifies an alkanoyl group of 1 to 6 carbons, in all of its isomericforms, or a carboxyalkanoyl group of 1 to 6 carbons, such as an oxalyl,malonyl, succinyl, glutaryl group, or a aromatic acyl group such asbenzoyl, or a halo, nitro or alkyl substituted benzoyl group. The word`alkyl` as used in the description or the claims, denotes astraight-chain or branched alkyl radical of 1 to 10 carbons, in all itsisomeric forms. Alkoxyalkyl protecting proups are groupings such asmethoxymethyl, ethoxymethyl, methoxyethoxymethyl, or tetrahydrofuranyland tetrahydropyranyl. Preferred alkylsilyl-protecting groups aretrimethylsilyl, triethylsilyl, t-butyldimethylsilyl, and analogousalkylated silyl radicals.

The present invention is more specifically described by the followingexamples, which are meant to be illustrative only of the process ofsynthesis and of the novel compounds obtainable thereby. In theseexamples, specific compounds identified by Arabic numerals (e.g.compounds 1, 2, 3, . . . etc.) refer to the structures so numbered inthe process schematics. Additionally, examples are provided illustrativeof the distinctive biological characteristics of the new compounds, suchcharacteristics serving as basis for the application of these compoundsas cell differentiation and antineoplastic agents.

PREPARATION OF COMPOUNDS General Procedures

Infrared spectra (IR) were obtained on a Nicolet MX-1 FT-IR spectrometerusing neat films of oily substances. Ultraviolet (UV) absorption spectrawere recorded with a Hitachi Model 60-100 UV-VIS spectrometer. Nuclearmagnetic resonance (NMR) spectra were recorded at 270 or 400 MHz withBruker WH-270 or AM-400 FT spectrometers in the solvents noted. Chemicalshifts (δ) are reported downfield from internal Me₄ Si (δ0.00) or CHCl₃(δ7.24). Low- and high-resolution mass spectra were recorded at 70 eV(unless otherwise stated) on a Kratos MS-50 TC instrument equipped witha Kratos DS-55 Data System. High resolution data were obtained by peakmatching. Samples were introduced into the ion source maintained at120°-250° C. via a direct-insertion probe. Silica gel 60 (Merck, 70-230or 230-400 mesh) was used for column chromatography. Thin-layerchromatography (TLC) was performed using precoated aluminum silica gelsheets with UV indicator from EM Science (Gibbstown, N.J.). Solventsystems used: A: chloroform-ethanol 85:15 (v/v); B: hexane-ethylacetatel:1; and C: hexane-ethyl acetate 3:1. High performance liquidchromatography (HPLC) was performed using a Waters Associates Liquidchromatograph equipped with a model 6000A solvent delivery system, aModel 6 UK Universal injector and a model 450 variable wavelengthdetector. Zorbax-Silica (Phenomenex) columns (6.2 mm×25 cm and 10 mm×25cm) were used. Solvent systems: A: 3% 2-propanol in hexane; B: 2%2-propanol inhexane; C: 6% 2-propanol in hexane; D: 10% 2-propanol inhexane; E: 20% 2-propanol in hexane; F: 2% ethyl acetate in hexane.Silica gel Sep-Pak (Waters Associates) cartridges were used for theprefiltration of HPLC samples. 3β-Acetoxy-22,23-bisnor-5-cholenic acidwas purchased from Steraloids (Wilton, N.H.). Tetrahydrofuran (THF) wasdistilled from sodium benzophenone ketyl. Other solvents were purifiedby standard methods. n-Butyllithium in hexane (Aldrich) was titratedwith n-propanol in the presence of 1,10-phenantroline in THF underargon. Reactions involving vitamin D compounds were carried out under anitrogen or argon atmosphere with magnetic stirring. Solutions andliquids were delivered by syringes through rubber septa.

EXAMPLE 1 Synthesis of 1α-hydroxyvitamin C-22 Ester (Compound 10) andHydroxy-protected Derivatives (Compounds 8, 11) (Process Scheme I) (a)Preparation of Steroid Esters 1 and 2

3β-Acetoxy-22,23-bisnor-5-cholenic acid (10 g) was dissolved in 420 mLof 5% KOH in methanol and the solution was stirred at ambienttemperature for 15 min to hydrolyze the acetate. To this solution, 160mL of 10% H₂ SO₄ in methanol was added dropwise with stirring and theresulting suspension was diluted with 400 mL of 1% H₂ SO₄ in methanol.The mixture was heated under reflux for 48 h to complete theesterification (checked by TLC, solvent system A). The product, themethyl ester (1) (9.0 g, 88%) was isolated by standard procedure, and4.4 g (12 mmol) of that material was dissolved in 135 mL of drydimethylformamide (DMF) and imidazole (3.6 g, 52.8 mmol) was added,followed by tert-butyldimethylsilyl chloride (4.0 g, 26.4 mmol). Thesolution was stirred at room temperature for 5 min until the bulkyprecipitate was formed and then stirring was continued for an additional15 min. The reaction mixture was extracted with hexane (400 mL), washedwith water, saturated NaCl solution, and dried over MgSO₄. Evaporationof the solvent provided TLC pure (solvent system b) silylated ester 2(5.3 g) as a colorless oil that was used for the next step withoutfurther purification. An analytical sample was obtained by flashchromatography using 2% ethyl acetate in hexane: IR (film) 1737.99,1604.89 cm⁻¹.

(b) Preparation of 5,7-Diene (3)

A mixture of compound (2) (1.0 g, 2.1 mmol), dibromantin (0.42 g, 1.5mmol) and anhydrous sodium bicarbonate (0.91 g, 10 mmol) in 20 mL ofhexane was heated under reflux in a nitrogen atmosphere for 30 min untilno starting ester 2 was detected (TLC, system C). The precipitate wasfiltered and the solution dried down under reduced pressure. The residuewas redissolved in 5 mL of anhydrous THF, tetrabutylammonium bromide(0.06 g, 0.19 mmol) was added and the mixture stirred at roomtemperature for 30 min under nitrogen. A solution of tetrabutylammoniumflouride (10 mL, 1M in THF) was then added followed by 0.7 mL ofs-collidine and the mixture was stirred under nitrogen at roomtemperature for 1 h. Another 5 mL of tetrabutylammonium flouridesolution was added and stirring was continued for 3 h. Ether (50 mL) wasadded and the organic phase was washed with water, cold 1N HCl, 10%NaHCO₃ and dried over anhydrous MgSO₄. The product, dissolved in benzenewas chromatographed on silica gel 70-230 mesh (30 g). Ester 3 (0.44 g,58%) was eluted using ethyl acetate in hexane. An analytical sample wasobtained by HPLC, solvent system A, R_(v) 77 mL: UV (3% 2-propanol inhexane), λ_(max) =262 nm (ε7,000), λ_(max) =272 nm (ε9,800), λ_(max)=282 nm (ε10,500), λ_(max) =293 (ε6,000); ¹ H NMR (CDCl₃) δ, 0.54 (s,3H, 18-CH₃), 0.94 (s, 3H, 19-CH₃), 1.22 (d, 2H, J=6 Hz, 21-CH₃), 3.6 (m,1H, 3-H), 3.68 (s, 3H, CO₂ CH₃), 5.42 (m, 1H, 6-H), 5.58 (m, 1H, 7-H);MS, m/z (relative intensity) M⁺ 358 (61), 340 (12), 325 (100), 299 (68),271 (7), 253 (17), 237 (26), 211 (27), 143 (72), 119 (35).

(c) Preparation of Vitamin Ester (4)

A solution of diene 3 (830 mg, 2.3 mmol) in 350 mL of benzene-ethylether, 1:4 (v/v) was irradiated with stirring under nitrogen in awater-cooled quartz immersion well equipped with a nitrogen bubbler anda Vycor filter using Hanovia 608A36 medium-pressure UV lamp for 40 min(4×10 min). The reaction was monitored by HPLC using 2% 2-propanol inhexane at 265 nm. Solution was dried down under reduced pressure,redissolved in 100 mL of absolute ethanol and heated under reflux in anitrogen atmosphere for 3 h. Then the solution was concentrated,redissolved in 1 mL of 10% ethyl acetate in hexane and chromatographedon silica gel 70-230 mesh (30 g). Vitamin ester 4 (298 mg, 36%) waseluted using a mixture of 15% ethyl acetate in hexane. An analyticalsample was obtained by HPLC, solvent system B, R_(v) 94 mL: IR (film)1738.95 cm⁻¹ ; UV, λ_(max) 264 nm; ¹ H NMR (CDCl₃), δ, 0.56 (3H, s,18-CH₃), 1.20 (3H, d, J=7 Hz, 21-CH₃), 3.66 (3H, s, CO₂ CH₃), 3.95 (1H,m, 3-H), 4.80 (1H, d, J=1.2 Hz, 19Z-H), 5.05 (1H, d, J=1.2 Hz, 19E-H),6.03 (1H, d, J=11 Hz, 7-H), 6.23 (1H, d, J=11 Hz, 6-H); MS, m/z(relative intensity), M⁺ 358 (45), 340 (9), 325 (45), 299 (22), 253(19), 237 (18), 136 (60), 118 (100).

(d) Preparation of 3,5-Cyclovitamin Ester (6)

Ester 4 was converted into tosylate 5 by the known method usingp-toluenesulfonyl chloride in pyridine at 4° C. for 20 h. Crude tosylate5 (102 mg, 0.2 mmol), dissolved in 2 mL of anhydrous dichloromethane,was added with stirring to a methanol solution (15 mL) containinganhydrous potassium bicarbonate (250 mg), at 55° C. The mixture wasstirred under nitrogen at 55° C. for 24 h. The solvents were thenremoved under reduced pressure and the residue extracted with ether. Theorganic phase was washed with water and dried over anhydrous MgSO₄. Theproduct, cyclovitamin ester 6, was purified by silica gel chromatographyusing 20% ethyl acetate in hexane (50 mg, 68%): ¹ H NMR (CDCl₃), δ0.54(3H, s, 18-CH₃), 0.74 (m, 3H), 0.91 (m, 4-H), 1.20 (3H, d, J=7 Hz,21-CH₃), 3.25 (3E, s, 6R-OCH₃), 3.65 (3H, s, 22-CO₂ CH₃), 4.15 (1H, d,J=9 Hz, 6-H), 4.88 (1H, 19E-H), 5.00 (1H, d, J=9 Hz, 7-H), 5.02 (1H,19E-H); MS, m/z (relative intensity) 372 (M⁺, 17), 340 (100), 253 (48),221 (40), 135 (72).

(e) Preparation of 5,6-Cis 1α-Hydroxyvitamin Ester (8) and Trans Isomer(9)

Tert-butyl hydroperoxide (112 μL, 3.0M solution in toluene, 0.34 mmol)was added to a suspension of selenium dioxide (9 mg, 0.8 mmol) in 2 mLof dry methylene chloride. The mixture was stirred at room temperatureunder nitrogen until a clear solution was formed. Anhydrous pyridine (12μL, 0.15 mmol) was then added followed by ester 6 (50 mg) dissolved in 2mL of anhydrous dichloromethane. The mixture was stirred under nitrogenfor 30 min. Cold 10% sodium bicarbonate (2 mL) was added and the mixtureextracted with ether. The organic phase was washed with cold 10% sodiumbicarbonate ice water and dried over anh. MgSO₄. Silica gelchromatography (10-20% ethyl acetate in hexane) afforded 12.5 mg of the1α-hydroxy compound (7). This product was then immediately dissolved in0.5 mL of glacial acetic acid and the solution was heated at 55° C. withstirring under nitrogen for 15 min. The reaction mixture was poured overice, extracted with ether and washed with ice-cold saturated sodiumbicarbonate. The combined ether extracts were washed with water anddried over anh. MgSO₄. Ester 8 was isolated (6 mg, 20% overall yieldfrom 5) by the general procedure described by DeLuca et al., U.S. Pat.No. 4,554,106. Analytical samples of (5Z,7E) and (5E,7E) isomers, 8 and9, respectively, were obtained by preparative HPLC in a ratio of 2.5:1.

Compound (8): HPLC, solvent system C, R_(v) 68 mL; UV (EtOH) λ_(max) 264nm, λ_(min) 227 nm, A264/A227=2.07; ¹ H NMR (CDCl₃) δ, 0.56 (3H, s,18-CH₃), 1.20 (3H, d, J=6.5 Hz, 21-CH₃), 2.04 (3H, s, 3β-acetyl), 3.66(3H, s, 22-CO₂ CH₃), 4.4 (1H, m, 1-H), 5.2 (1H, m, 3-H), 5.01 (19E-H),5.34 (19Z-H), 6.01 (1H, d, J=10 Hz, 7-H), 6.33 (1H, d, J=10 Hz, 6-H),MS, m/z (relative intensity), 416 (M⁺, 4), 356 (100), 338 (21), 251(13), 134 (95).

Compound (9): HPLC, solvent system C, R_(v) 78 ml; UV (EtOH), λ_(max)267 nm, λ_(min) 227 nm, A267/A227=3.51; ¹ H NMR (CDCl₃) δ, 0.56 (3H, s,18-CH₃), 1.20 (3H, d, J=6.5 Hz, 21-CH₃), 2.04 (3H, s, 3β-OAc), 3.66 (3H,s, 22-CO₂ CH₃), 4.5 (1H, m, 1-H), 5.3 (1H, m, 3-H), 4.99 (19E-H), 5.13(19Z-H), 5.81 (1H, d, J=10 Hz, 7-H), 6.56 (1H, d, J=10 Hz, 6-H).

(f) Preparation of 1α-Hydroxy Ester (10) and Disilyl Ester (11)

A 0.1N solution of KOH in methanol (10 mL) was added to a stirredsolution of acetate ester 8 (100 mg, 0.24 mmol) in ethyl ether (10 mL).The resulting solution was stirred at room temperature for 90 min untilno starting material was detected by TLC (solvent system B).Dihydroxyester (10) was isolated by standard extraction procedure (ethylacetate, saturated NaCl, anhydrous MgSO₄).

A mixture of imidazole (250 mg, 3.6 mmol) and tert-butyldimethylsilylchloride (250 mg, 1.6 mmol) in DMF (2 mL) was added to a stirredsolution of ester (10) (86.2 mg, 0.23 mmol) in 4 mL of DMF. Theresulting homogenous mixture was stirred for 15 min at 55° C. until nostarting material was detected by TLC (solvent system B). The productwas isolated with hexane and the organic extract was washed with brineand dried over anhydrous MgSO₄. A hexane solution of the crude productwas filtered through silica gel Sep-Pak cartridge to give thedisilylated ester (11) (136 mg, 98%): UV (hexane), λ_(max) 264 nm,λ_(min) 227 nm, A264/A227=1.91; ¹ H NMR (CDCl₃), δ0.07 [12H, s, Si(CH₃)₂], 0.55 (3H, s, 18-CH₃), 0.86 [18H, s, C(CH₃)₃ ], 1.20 (3H, d, J=6.8 Hz,21-CH₃), 3.65 (3H, s, O-CH₃), 4.18 (1H, m, 3-H), 4.36 (1H, m, 1-H), 4.83(1H, d, J=1.2 Hz, 19Z-H), 5.16 (1H, d, J=1.2 Hz, 19E-H), 5.96 (1H, d,J=11.2 Hz, 7-H), 6.19 (1H, d, J=11.2 Hz, 6-H); MS, m/z (intensitiesnormalized to m/e 248), 602 (M⁺, 10), 470 (59), 413 (7), 338 (10), 248(100).

EXAMPLE 2 Synthesis of C-22-Alcohol (12) and C22-Tosyl Derivative (13)(Process Scheme II) (a) Preparation of C-22-Alcohol (12)

Lithium aluminum hydride (25 mg, 0.65 mmol) was added to stirredsolution of silyl ester (11) (136.2 mg, 0.23 mmol) in anhydrous THF (5mL) under argon at 0° C. The suspension was stirred for 15 min at 0° C.and the excess of LiAlH₄ was decomposed by the dropwise addition of 10%H₂ O in THF. The suspension was diluted with 10 mL of THF and thestirring was continued for an additional 15 min at room temperature. Theproduct was isolated by the standard extraction with ethyl acetate andsilica gel Sep-Pak filtration in 10% ethyl acetate in hexane. Disilylalcohol 12 was obtained as a colorless oil (118.4 mg) in 91% yield: UV(EtOH), λ_(max) 264, λ_(min) 227, A264/A227=1.57; ¹ H NMR (CDCl₃) δ0.00(12H, s, Si--CH₃) 0.53 (3H, s, 18-CH₃), 0.85 [18H, s, Si--C(CH₃)₃ ],1.04 (3H, d, J=6.4 Hz, 21-CH₃), 3.37 and 3.63 (1H and 1H, each m,22-CH₂), 4.17 (1H, m, 3-H), 4.35 (1H, m, 1-H), 4.84 (1H, brs, 19Z-H),5.16 (1H, brs, 19E-H), 6.0 (1H, d, J=12.2 Hz, 7-H), 6.21 (1H, d, J=12.2Hz, 6-H); MS, m/z (intensities normalized to m/e 248), 574 (M⁺, 17), 442(67), 383 (11), 308 (17), 248 (100).

(b) Preparation of 22-Alcohol-Tosylate (13)

An ice-cold solution of p-toluenesulfonyl chloride (42.7 mg, 0.22 mmol)in dry pyridine (50 μL) was added to a stirred solution of alcohol 12 at0° C. under nitrogen. The mixture was stirred at 5° C. for 22 h until nostarting material was detected by TLC using the solvent system C. Thereaction mixture was poured over ice-cold saturated aqueous NaHCO₃ andstirring was continued for another 30 min. The product was extractedwith ethyl ether-hexane 1:1 (v/v). The organic phase was washed withsaturated NaCl and dried over MgSO₃. Solvents were removed under reducedpressure and pyridine was removed in a stream of nitrogen. Crude productwas purified by silica gel Sep-Pak filtration (5% ethyl acetate inhexane) to give pure tosylate 13 (54 mg, 98%): IR (film) 3500, 2950,1580, 1367, 1267, 1189, 1178, 1099, 1085, 835 cm⁻¹ ; UV (hexane) λ_(max)263 nm, λ_(min) 236 nm; ¹ H NMR (CDCl₃) δ0.00 (12H, s, Si--CH₃), 0.43(3H, s, 18-CH₃), 0.81 [18H, s, Si--C(CH₃)₃ ], 0.93 (3H, d, J=6.8 Hz,21-CH₃), 2.40 (3H, s, Ar--CH₃), 3.64 and 3.91 (1H and 1H, each m,22-CH₂), 4.13 (1H, m, 3-H), 4.31 (1H, m, 1-H), 4.79 (1H, brs, 19Z-H),5.13 (1H, brs, 19E-H), 5.94 (1H, d, J=12.8 Hz, 7-H), 6.17 (1H, d, J=12.8Hz, 6-H), 7.43 and 7.84 (2H and 2H, each m, Ar--H); MS m/z (intensityrelative to m/z 248), 728 (6), 596 (30), 556 (7), 464 (7), 424 (44), 367(19), 292 (23), 248 (100); exact mass calcd. for C₄₁ H₆₈ O₅ Si₂ S728.4338, found 728.4326.

EXAMPLE 3 Synthesis of Phenylsulfone Side Chain Units (Process SchemeIII) (a) Preparation of Phenylsulfone Side Chain Residue 18

A solution of 4-chlorovaleryl chloride 14 (Aldrich; 3 g, 19.2 mmol) inanhydrous THF (25 mL) was added dropwise with vigorous stirring, over 30min, under argon, to a solution of methylmagnesium bromide (12.9 mL of a3M solution in ether) in 25 mL of dry THF at -10° C. The reactionmixture was then allowed to warm up to room temperature within 2 h, thenquenched with water and neutralized with diluted hydrochloric acid. Themixture was extracted with ether, the combined organic layers werewashed with water and dried with sodium sulfate. After removal of thesolvent, the residue was distilled in vacuo to give chloroalcohol 15 asa colorless liquid (2.1 g, 70%). Product 15 (1.5 g, 10 mmol) inanhydrous dimethylformamide (5 mL) was then added to a stirred solutionof thiophenol (1.32 g, 12 mmol) and potassium t-butoxide (1.32 g, 11.3mmol) in anhydrous dimethylformamide (25 mL). The reaction mixture wasstirred at room temperature overnight and the solution was partitionedbetween dichloromethane and water. The organic layer was washed withaqueous sodium carbonate, water and dried over anhydrous magnesiumsulfate. The solvent was evaporated in vacuo and the crude oil waspurified by silica gel flash chromatography with hexane-ethyl acetate.Sulfide 16 (2.2 g, 98%) was obtained as a colorless liquid. Sulfide 16(1.01 g, 4.5 mmol) was then dissolved in dry dichloromethane (40 mL) and3-chloroperbenzoic acid (2.5 g, 11.6 mmol; Aldrich 80-85%) was added inportions with stirring and occasional cooling. The reaction mixture wasstirred for 2 h and then quenched with 10% sodium bicarbonate. Thecombined organic extracts were washed with aqueous sodium sulfite andbrine and dired over magnesium sulfate. The solvent was removed in vacuoand the crude oil was purified by silica gel flash chromatography usinghexane-ethyl acetate mixtures to afford sulfone 17 (1.1 g, 97%) as acolorless liquid. To a stirred solution of sulfone 17 (1.3 g, 5.1 mmol)and imidazole (1.5 g, 22.7 mmol) in dry dimethylformamide (50 mL),triethylsilyl chloride (1.15 g, 7.7 mmol) was added. The reactionmixture was kept at room temperature for 2 h and then diluted withdichloromethane. The mixture was washed with aqueous ammonium chloridesolution and water. The organic layers were dried over sodium sulfateand the solvent removed in vacuo. The residue was purified by silica gelflash chromatography. Hexaethyldisiloxane was first eluted with hexane.The triethylsilyl-protected sulfone 18 (1.8 g, 97%) was eluted withhexane-ethyl acetate 9:1 as a colorless liquid: IR (neat): 3045, 2940,1440, 1360, 1130, 1020 cm⁻¹ ; ¹ H NMR (400 MHz, CDCl₃) δ0.518 (6H, q,J=6.2 Hz, Si--CH₂), 0.899 (9H, t, J=6.2 Hz, Si--C--CH₃), 1.142 (6H, s,CH₃), 1.307-1.462 (4H, m), 1.655-1.738 (2H, m, H-4), 3.080-3.122 (2H, m,H-2), 7.567 (2H, t, J=6.8 Hz, H-aryl meta), 7.648 (1H, t, J=6.8 Hz,H-aryl para), 7.916 (2H, d, J=6.83 Hz, H-aryl ortho); MS (EI, 7 eV): m/z(relative intensity) 372 (M⁺, 2), 341 (100 ), 229 (2), 227 (18), 173(24), 103 (22), 75 (45), 55 (33).

As shown by the illustrative examples of Process Scheme III, otherphenylsulfone units can be prepared by the general method describedabove, or by analagous methods described in the literature. For example:

(b) Preparation of Phenylsulfone (19)

By treatment of dichloro compound 14 according to the proceduredescribed in Example 3(a) above, but substituting ethylmagnesium bromidefor methylmagnesium bromide in the first reaction step, there isobtained the phenylsulfone homolog of structure 19 (Z=Et₃ Si).

(c) Preparation of Phenylsulfone (23)

A solution of 6-bromohexanoyl chloride (20, 3.8 g, 2.8 mL, 18 mmol) inanhydrous tetrahydrofuran (10 mL) was added dropwise with vigorousstirring over 15-20 min under argon atmosphere to a solution ofmethylmagnesium bromide (14 mL of 3M solution in ether) in anhydroustetrahydrofuran (15 mL) at -10° C. The mixture was stirred at roomtemperature for 2 h, cooled to 0° C. and carefully decomposed with 1:1diluted hydrochloric acid. The mixture was extracted with ether, thecombined organic layers were washed with water, dried over anhydrousmagnesium sulfate and evaporated to give the bromo alcohol (21) as acolorless oil (3.6 g, 94%).

The bromo-alcohol (3.4 g, 16 mmol) was treated with benzene sulfinicacid sodium salt (3.3 g, 20 mmol) in anhydrous dimethylformamide at 70°C. for 41/2 h. The mixture was poured on ice, extracted withdichloromethane, washed with 1N HCl, water, 10% NaHCO₃ solution, driedover anhydrous MgSO₄, filtered and evaporated to give the sulfone (22),which was purified by flash chromatography on silica gel and eluted with40-50% ethyl acetate in hexane to give the sulfone containing some ofthe corresponding sulfinate ester (4.18 g, 98%), MS, m/z (270 (M⁺), 255(M⁺ -15), 77, 59.

To a stirred solution of the sulfone (22) (4 g, 14 mmol) and imidazole(3.8 g, 55 mmol) in anhydrous dimethylformamide (13 mL) triethylsilylchloride (4.6 g, 5.1 mL, 30 mmol) was added. The reaction mixture wasstirred at room temperature for 2 h, poured on ice water, extracted withether, dried over anhydrous MgSO₄, filtered and evaporated. The residuewas purified by flash chromatography. Hexaethyldisiloxane was firsteluted with hexane; 3% ethyl acetate in hexane eluted the protectedsulfinate ester with some of the sulfone, and 10% ethyl acetate inhexane eluted the protected pure sulfone (23) (3.4 g, 60%). Anal. calcd.for C₂₀ H₃₆ O₃ SSi C, 62.45%, H, 9.43%, S 38.34%, Found C, 61.97%, H,9.45%, S, 8.33%. MS, m/z (relative intensity) 355 (100) (M⁺ -29), 227(15), 173 (35), 103 (43), 75 (95), 55 (23), NMR (400 MHz, CDCl₃), 0.54(6H, q, J=7 Hz, Si--CH₂), 0.94 (9H, t, J=8 Hz), Si--C--CH₃), 1.15 (6H,s, CH₃), 1.31-1.36 (4H, m), 3.08-3.12 (2H, m, H-2), 7.57 (2H, t, J=6.8Hz, H-aryl-meta), 7.66 (1H, t), H-aryl para), 7.92 (2H, d, J=6.8 Hz,H-aryl ortho).

(d) Preparation of Phenylsulfone (26)

Treatment of the commercially available 3-phenylsulfonylpropionic acid(24) with acidic methanol under standard esterification conditionsprovides the corresponding methyl ester, which is then subjected to aGrignard reaction using methyl magnesium bromide to obtain thephenylsulfone derivative (25). The reaction of (25) withtriethylsilylchloride, using the conditions of Example 3(a) above, thengives the hydroxy-protected sulfone (26).

(e) Preparation of Sulfone Unit (28)

By application of the procedure given in Example 3(d) to acid (24), butsubstituting ethylmagnesium bromide for methylmagnesium bromide in theGrignard reaction step, there is obtained the protected-sulfone homolog(28).

As illustrated by the above examples, the phenylsulfone units arepreferably prepared, and subsequently used, as hydroxy-protectedderivatives. In addition to triethylsilyl-protecting groups, otherpreferred hydroxy-protecting groups are t-butyldimethylsilyl,tetrahydrofuranyl, and tetrahydropyranyl.

EXAMPLE 4 Side Chain Coupling Reaction (Process Scheme IV) (a)Preparation of Vitamin Sulfones 29 and 30

A solution of 1,10-phenanthroline (used as indicator) in anhydrous THFwas added under argon to the 1.35M solution of n-BuLi in hexane (48 μL,64 μmol) to obtain a dark-red color of the mixture. The solution wasplaced in an acetone-dry ice bath and diisopropylamine (9 μL, 64 μmol)was added. The resulting solution was stirred under argon for 30 min at-77° C. Then the solution of sulfone 18 (29 mg, 80 μmol) in 100 μL ofTHF was added followed by another 100 μL of THF used in rinsing. Theresulting brown mixture was stirred at -75° C. under argon for 30 minand the cooling bath was replaced with CCl₄ dry ice bath. After 15 minof stirring at -21° C. the solution of tosylate 13 (11.6 mg, 16 μmol)was added and the color of the reaction mixture turned black to red. Thesolution was stirred at -20° to -10° C. for 3.5 h and 1 mL of saturatedNH₄ Cl was added at -10° C. The mixture was extracted with hexane andthe organic phase was washed with saturated NaCl.

The organic extract was filtered through silica gel Sep-Pak cartridgefollowed by 20 mL of 10% ethyl acetate in hexane. Preparative HPLC(column 6.2×25 cm) with solvent system F provided at R_(v) 37 mL theunreacted tosylate 13 (3.0 mg). Sulfone 29 (2.1 mg, 19%) was then elutedat R_(v) 55 mL: IR (film) 3500, 2956, 1440, 1301, 1258, 1147, 1086,1072, 1064 cm⁻¹ ; UV (hexane) λ_(max) 264 nm, λ_(min) 230 nm,A264/A230=1.96; ¹ H NMR (CDCl₃) δ0.41 (3H, s, 18-CH₃), 0.51 (6H, q,J=5.7 Hz, Si--CH₂ --), 0.86 and 0.88 [9H and 9H, each, s, Si--C(CH₃)₃ ],0.90 (9H, t, J=8 Hz, SiCH₂ --CH₃), 1.13 (3H, d, J=5.8 Hz, 21CH₃), 1.23(6H, s, 26,27-CH₃), 4.17 (1H, m, 3-H, 4.37 (1H, m, 1-H), 4.85 (1H, brs,19Z-H), 5.17 (1H, brs, 19E-H), 5.99 (1H, d, J=11.0 Hz, 7-H), 6.21 (1H,J=10.8 Hz, 6-H), 7.54 (2H, t, J=7.3 Hz, Ar--H, meta), 7.61 (1H, t, J=7.3Hz, Ar--H, para), 7.33 (2H, d, J=7.3 Hz, Ar--H, ortho); MS, m/z(relative intensity), 926 (M⁺, 16), 794 (100), 737 (9), 530 (9), 521(6), 389 (13), 301 (8).

Sulfone 30 (3.8 mg, 35%) (epimer of 29 at carbon 23) was eluted at R_(v)87 mL: IR (film) 3500, 2955, 1440, 1304, 1257, 1148, 1086, 1072, 1064cm⁻¹ ; UV (hexane) λ_(max) 264 nm, λ_(min) 229 nm, A264/A229=2.06; ¹ HNMR (CDCl₃) δ0.49 (3H, s, 18-CH₃), 0.51 (6H, q, J=5.7 Hz, Si--CH₂ --),0.85 [18H, s, Si--C(CH₃)₃ ], 0.90 (9H, t, J=7.9 Hz, Si--CH₂ --CH₃), 1.13(3H, d, J=6.2 Hz, 21-CH₃), 1.23 (6H, s, 26,27-CH₃), 4.16 (1H, m, 3-H),4.35 (1H, m, 1-H), 4.83 (1H, brs, 19Z-H), 5.16 (1H, brs, 19E-H), 5.98(1H, d, J=Hz, 7-H), 6.20 (1H, d, J=11.3 Hz, 6-H), 7.54 (2H, t, J=7 Hz,Ar-meta), 7.61 (1H, t, J=7 Hz, Ar--H, para), 7.86 (2H, d, J=7 Hz, Ar--H,ortho); MS m/z (relative intensity), 926 (19), 794 (100), 737 (11), 530(28), 521 (14), 389 (33), 301 (14).

(b) 24-dihomo-1,25-dihydroxyvitamin D₃ (32)

A saturated solution of Na₂ HPO₄ in methanol (0.5 mL) was added to astirred solution of sulfone 29 (1.80 mg) in 0.5 mL of anhydrous THFfollowed by powdered anhydrous Na₂ HPO₄ (80 mg). The mixture was stirredunder argon for 30 min and cooled down to 0° C. Fresh 5% sodium amalgam(ca. 200 mg) was then added and the mixture was stirred for 3 h at 5° C.until no starting material was detected by TLC (solvent system C). Themixture was diluted with hexane (3 mL) and stirring was continued for 15min. Solvents were decanted and the solid materials were washed withhexane (3×2 mL). Ice and saturated NaCl (2 mL) was added to the combinedorganic solutions. The organic layer was washed with saturated NaCl andfiltered through silica Sep-Pak cartridge to give the hydroxy-protected24-dihomo-1α, 25-dihydroxyvitamin D₃ compound 31 (1.19 mg, 1.5 μmol,78%) as a colorless oil. Compound 31 was also obtained the same way bythe sodium amalgam reduction of sulfone 30. Thus, sulfones 29 and 30, asobtained in Example 4(a) above, need not be separated prior to thesodium amalgam reduction. A mixture of both can be effectively reducedto obtain the desired vitamin D derivative 31.

Compound 31 (1.1 mg) was dissolved in 0.5 mL of anhydrous THF and tothis solution tetrabutylammonium fluoride in THF (20 μL, 1M solution)was added. The mixture was stirred under argon for 50 min at 50° C.Ether (3 mL) was then added and the organic phase was washed withsaturated NaCl. Solvents were removed and the residue was dissolved in10% 2-propanol in hexane and filtered through silica Sep-Pak.Preparative HPLC (solvent system D, column 6.2 mm×25 cm, R_(v) 62 mL)yielded the desired vitamin D homolog 32 (465 μg, 76%); IR (film) 3360,2927, 1602, 1447, 1376, 1297, 1146, 1106, 1086, 1064 cm⁻¹ ; UV (10%2-propanol in hexane) λ_(max) 264 nm, λ_(min) 228 nm, A264/A228=1.91; ¹H NMR (CDCl₃) δ0.52 (3H, s, 18-CH₃), 0.90 (3 H, d, J=6.4 Hz, 21-CH₃),1.19 (6H, s, 26,27-CH₃), 4.22 (1H, m, 3-H), 4.42 (1H, m, 1-H), 4.99 (1H,brs, 19Z-H), 5.31 (1H, brs, 19E-H), 6.00 (1H, d, J=11.1 Hz, 7-H), 6.36(1H, d, J=11.2 Hz, 6-H); MS m/z (relative intensity) 444 (M⁺, 1.4), 426(41), 393 (10), 251 (26), 209 (17), 197 (20), 157 (29), 155 (37), 134(58), 105 (54), 59 (100); exact mass calcd. for C₂₉ H₄₈ O₃ 444.3603,found 444.3609.

(c) Side Chain Coupling Using Phenyl Sulfone (23)

Vitamin tosylate 13 is reacted with the phenylsulfone units 23 under theconditions described in Example 4(a) above, to obtain the correspondingvitamin sulfone adduct. After Na/Hg reduction of that adduct, using thegeneral conditions of Example 4(b) above, and subsequent removal of thehydroxy-protecting groups as described in Example 4(b), there isobtained the desired product, 24-trihomo-1,25-dihydroxyvitamin D₃,characterized by the structure below: ##STR7##

BIOLOGICAL ACTIVITY OF 24-DIHOMO-1,25-DIHYDROXYVITAMIN D₃ (COMPOUND 32)EXAMPLE 5 Measurement of Differentiation in HL-60 Cells (Table 1)

The measurement of differentiation in HL-60 cells (human leukemia cells)was carried out according to the general procedures described by DeLucaet al., U.S. Pat. No. 4,717,721. In addition to the methods describedtherein, nonspecific acid esterase activity was measured as described bythe kit marketed by the Sigma Chemical Corporation of St. Louis, Mo. Asshown in Table 1, degree of differentiation is assessed by threedifferent assays (NBT reduction, phagocytosis, and esterase activity)and results are expressed as the percent of differentiated cellsproduced in response to treatment with various concentrations of vitaminD compounds.

                  TABLE 1                                                         ______________________________________                                        Differentation Activity of 24-Dihomo-1,25-(OH).sub.2 D.sub.3                  in HL-60 Cells in Culture                                                             Concen-                                                                              % Cells Showing Differentation                                           tration              Phago-                                         Compound  (molar)  NBT Reduction                                                                             cytosis                                                                              Esterase                                ______________________________________                                        1,25-(OH).sub.2 D.sub.3                                                                 1 × 10.sup.-9                                                                    .sup. 32 ± 3.sup.a                                                                     .sup. 28 ± 3.sup.a                                                                .sup. 28 ± 2.sup.a                             1 × 10.sup.-8                                                                    51 ± 3   56 ± 3                                                                            60 ± 4                                         1 × 10.sup.-7                                                                    82 ± 4   84 ± 4                                                                            86 ± 2                               24-Dihomo-                                                                              1 × 10.sup.-9                                                                    28 ± 3   36 ± 3                                                                            35 ± 4                               1,25-(OH).sub.2 D.sub.3                                                                 5 × 10.sup.-9                                                                    64 ± 4   62 ± 4                                                                            62 ± 3                               (compound 32)                                                                           1 × 10.sup.-8                                                                    70 ± 2   68 ± 2                                                                            68 ± 4                                         5 × 10.sup.-8                                                                    93 ± 5   94 ± 2                                                                            94 ± 2                               ______________________________________                                         .sup.a Standard error of the mean of 3-4 determinations.                 

The results of these three assays are shown in Table 1. It is evidentthat the novel homolog 24-dihomo-1,25-(OH)₂ D₃ (compound 32) is muchmore active than 1,25-(OH)₂ D₃ itself in causing differentiation ofHL-60 cells in culture (Table 1). Thus, 1,25-(OH)₂ D₃, the naturalhormone, induces the differentiation of about 50-60% of the cells at aconcentration of 1×10⁻⁸ molar, whereas the new 24-dihomo analog(compound 32) gives over 60% differentiation at five-fold lowerconcentration (5×10⁻⁹ M). Similarly, a concentration of 1×10⁻⁷ molar isrequired for 1,25-(OH)₂ D₃ to produce about 80% differentiated cells,but the dihomo analog gives better than 90% differentiation at 5×10⁻⁸ M(e.g. 5× lower). These results strongly support the conclusion that the24-dihomo-1,25-(OH)₂ D₃ is 5-10 times more active than 1,25-(OH)₂ D₃ incausing differentiation of HL-60 cells in culture.

EXAMPLE 6 Assay for Calcemic Activity in the Rat (a) Intestinal CalciumTransport Activity and Mineralization of Rachitic Rat Bone (Table 2)

Male weanling rats were obtained from the Harlan-Sprague Dawley Companyof Madison, Wis. and fed the high calcium, low phosphorus rachitogenicdiet described by Suda et al. (J. Nutr. 100, 1049-1052, 1970). They werefed on this diet for a total of 4 weeks ad libitum. At the end of thethird week the animals were divided into groups of 6 rats each. Onegroup received a daily injection of vehicle (0.1 ml of 95% propyleneglycol, 5% ethanol) interperitoneally. The remaining groups received thesame amount of vehicle but containing one of the following doses: 12.5ng 1,25-(OH)₂ D₃ ; 25 ng 1,25-(OH)₂ D₃ ; 12.5 ng 24-dihomo-1,25-(OH)₂ D₃; or 25 ng 24-dihomo-1,25-(OH)₂ D₃. The animals were killed 24 hoursafter the last dose, the intestines removed, and the duodenal segmentswere used to measure intestinal calcium transport as described byHalloran and DeLuca (Arch. Biochem. Biophys. 208, 477-486, 1981).Calcium transport activity is presented in Table 2 as the calciumtransport ratio, indicated by an I/O notation [the concentration ofcalcium in the serosal medium (I) over the calcium concentration in themucosal medium (O)]. To assay mineralization of bone in response to testcompounds, the femurs of all animals were removed, extracted for 24hours with 95% ethanol in a Soxlet extractor and 24 hours withchloroform using a Soxhlet extractor. They were dried to constant weightand the total as well as percent ash determined in the femurs followingashing at 600° F. for 24 hours. The calcium transport ratio is indicatedby an I/O notation [the concentration of calcium in the serosal medium(I) over the calcium concentration in the mucosal medium (O)]. The ashcontent is measured in total milligrams of ash/femur or the percent ashbased on a defatted dry bone weight.

                  TABLE 2                                                         ______________________________________                                        Response of Rachitic Rats to                                                  24-Dihomo-1,25-Dihydroxyvitamin D.sub.3                                                 Calcium Trans-                                                                             Femur Ash  Femur Ash                                             port Ratio I/O                                                                             mg (mean ±                                                                            % (mean ±                                Group     (mean ± S.E.M.)                                                                         S.E.M.)    S.E.M.)                                     ______________________________________                                        Control   .sup. 4.0 ± 0.2.sup.a                                                                   .sup. 17.7 ± 1.0.sup.a                                                                .sup. 14.0 ± 0.20.sup.a                  (Vehicle)                                                                     1,25-(OH).sub.2 D.sub.3                                                       12.5 ng/day                                                                             10.4 ± 0.8                                                                              26.3 ± 1.6                                                                            18.9 ± 0.52                              25 ng/day 13.2 ± 1.0                                                                              26.4 ± 1.7                                                                            19.9 ± 1.3                               24-Dihomo-                                                                    1,25-(OH).sub.2 D.sub.3                                                       (compound 32)                                                                 12.5 ng/day                                                                             3.0 ± 0.3 21.0 ± 1.0                                                                            14.75 ± 1.4                              25 ng/day 3.0 ± 0.4 18.7 ± 1.5                                                                            14.8 ± 0.69                              ______________________________________                                         .sup.a Standard error of the mean of 6 determinations.                   

(b) Measurement of Intestinal Calcium Transport and Bone CalciumMobilization (Tables 3 and 4)

Male weanling rats were obtained from the Harlan Sprague Dawley Companyand fed the low calcium vitamin D-deficient diet described by Suda etal. (J. Nutr. 100, 1049-1052, 1970) for a period of 4 weeks. At the endof the third week the animals received the indicated doses (Tables 3 and4) dissolved in 95% propylene glycol and 5% ethanol. Each animalreceived 0.1 ml of solvent vehicle containing the indicated dosage eachday for 7 days; the control group received solvent only. Intestinalcalcium transport was measured as described by Halloran and DeLuca(Arch. Biochem. Biophys. 208, 477-486, 1981) and serum calcium wasmeasured using an atomic absorption spectrophotometer (U.S. Pat. No.4,717,721).

                  TABLE 3                                                         ______________________________________                                        Response of Rats on a Low Calcium                                             Diet to 24-Dihomo-1,25-Dihydroxyvitamin D.sub.3                                              Calcium Trans-                                                                             Serum Calcium                                                    port I/O     mg %                                              Group          (mean ± S.E.M.)                                                                         (mean ± S.E.M.)                                ______________________________________                                        Control        .sup.  4.8 ± 0.26.sup.a                                                                 .sup. 4.1 ± 0.05.sup.a                         1,25-(OH).sub.2 D.sub.3                                                       12.5 ng/day    11.2 ± 0.6                                                                              4.8 ± 0.08                                     25 ng/day      13.2 ± 1.2                                                                              4.8 ± 0.08                                     24-Dihomo-1,25-(OH).sub.2 D.sub.3                                             (compound 32)                                                                 125 ng/day     9.4 ± 0.8 4.2 ± 0.06                                     ______________________________________                                         .sup.a Standard error of the mean of 6 determinations.                   

                  TABLE 4                                                         ______________________________________                                        Response to Serum Calcium of Vitamin D-Deficient Rats on                      a Low Calcium Diet to 24-Dihomo-1,25-Dihydroxyvitamin D.sub.3                                   Serum Calcium                                                                 mg %                                                        Group             (mean ± S.E.M.)                                          ______________________________________                                        Control           .sup.  3.4 ± 0.07.sup.a                                  1,25-(OH).sub.2 D.sub.3                                                       12.5 ng/day/7 days                                                                               3.7 ± 0.17                                              25 ng/day          4.1 ± 0.07                                              75 ng/day          4.6 ± 0.09                                              24-Dihomo-1,25-(OH).sub.2 D.sub.3                                             (compound 32)                                                                 25 ng/day          3.6 ± 0.16                                              125 ng/day        3.95 ± 0.13                                              250 ng/day        3.80 ± 0.05                                              ______________________________________                                         .sup.a Standard error of the mean of 6 determinations.                   

The results shown in Tables 2 and 3 illustrate that 24-dihomo-1,25-(OH)₂D₃ (compound 32) is much less active than 1,25-(OH)₂ D₃ in stimulatingintestinal calcium transport. 1,25-(OH)₂ D₃ achieves maximal calciumtransport at between 12.5 and 25 ng per day, whereas24-dihomo-1,25-(OH)₂ D₃ showed no activity whatever at these doses(Table 2), and did produce a significant response only at the 125 ng perday level (Table 3). Even then, the response was lower than thatproduced by 1,25-(OH)₂ D₃ at 12.5 ng/day. These results show the dihomoanalog to have one-tenth or less of the activity of 1,25-(OH)₂ D₃ instimulating intestinal calcium transport (Tables 2 and 3).

In the case of bone mineralization (Table 2), a similar lack ofeffectiveness of 24-dihomo-1,25-(OH)₂ D₃ on mineralization of theskeleton is observed. The entries under Femur Ash and Femur Ash % inTable 2 show that doses of 12.5 and 25 ng of the dihomo compound (32)produce no significant change compared to the control, whereas1,25-(OH)₂ D₃ elicits significant mineralization at these same dosagelevels.

When bone calcium mobilization (serum calcium levels) was measured, itis clear that 1,25-(OH)₂ D₃ produced a significant elevation of serumcalcium at the expense of bone at doses of 12.5 and at 25 ng per day(Tables 3 and 4). In contrast, 24-dihomo-1,25-(OH)₂ D₃ elicited nosignificant response at a dosage level of 25 ng/day and none at 125ng/day in one experiment (Table 3), and a very modest rise in serumcalcium when administered at levels of 125 and 250 ng/day in a secondexperiment (Table 4). Notable, too, is the observation (Table 4) thatincreasing the dose from 125 to 250 ng/day does not further increaseblood calcium levels. These results indicate that the24,24-dihomo-1,25-(OH)₂ D₃ is less than one-tenth as active as1,25-(OH)₂ D₃ in raising serum calcium at the expense of bone. Theseresults are confirmed also by a third experiment measuring bone calciummobilization, in which the dihomo analog 32 when tested over a dosagerange up to 1000 ng/day elicited no response.

Thus, it is evident that the new vitamin D homolog (compound 32) showsunexpected preferential activity in cellular differentiation whilehaving little effect on calcium transport and mobilization. This is thetype of activity pattern desired for a vitamin D compound intended foruse as an anticancer agent. By virtue of its very low calcemic action(compared to 1,25-(OH)₂ D₃), the new dihomo-1,25-(OH)₂ D₃ analog can beadministered without inducing an undesired hypercalcemic response inpatients, while exhibiting a potency even higher than that of 1,25-(OH)₂D₃ in arresting malignant cell proliferation and in inducing celldifferentiation. The same type of activity pattern, namely pronounceddifferentiation activity combined with very low or abolished calcemicpotency, can be expected for the 24-trihomo analog of this invention,which compound, therefore, will also exhibit a greatly enhanced andadvantageous differentiation/calcemic activity ratio. Althoughstructurally related to 24-homo-vitamin D analogs of the prior art (U.S.Pat. No. 4,717,721), the side chain homovitamin D compounds of thisinvention present a radically altered activity profile: high activity incell differentiation combined with nearly abolished calcemic potency.Furthermore, since the differentiation activity is expressed in the caseof human leukemia cells (HL-60 cells), it is clear that these newvitamin homologs can be used for the treatment of malignancies inhumans, specifically leukemias.

For treatment purposes, these compounds can be formulated as solutionsin innocuous solvents, or as emulsions, suspensions or dispersions insuitable and innocuous solvents or carriers, or as pills, tablets orcapsules by conventional methods known in the art. Such formulations mayalso contain other pharmaceutically-acceptable and non-toxic excipients,such as stabilizers, anti-oxidants, binders, coloring agents oremulsifying or taste-modifying agents.

The compounds are advantageously administered by injection, or byintravenous infusion of suitable sterile solutions, or in the form oforal doses via the alimentary canal. For the treatment of humanleukemia, the homovitamin D compounds of this invention are administeredto subjects in dosages sufficient to induce the differentiation ofleukemic cells to macrophages. Preferred dosage amounts are from 0.5 μgto 50 μg per day, it being understood that dosages can be adjusted tostill higher levels depending on the severity of the disease or theresponse or the condition of subject as well-understood in the art.##STR8##

We claim:
 1. A compound of the structure:where X, Y and Z, which may bethe same or different, are selected from the group consisting ofhydrogen and a hydroxy-protecting group, and where n is 3 or 4, andwhere m and p are integers having, independently, values of 1 to
 5. 2. Acompound of claim 1 where X, Y, and Z are hydroxy-protecting groups. 3.A compound of claim 2 where X, Y, and Z are alkylsilyl groups.
 4. Acompound of claim 1 where X, Y, and Z are hydrogen. 5.26,27-dimethyl-24-dihomo-1α,25-dihydroxyvitamin D₃. 6.26,27-dimethyl-24-trihomo-1α,25-dihydroxyvitamin D₃. 7.26,27-diethyl-24-dihomo-1α,25-dihydroxyvitamin D₃. 8.26,27-diethyl-24-trihomo-1α,25-dihydroxyvitamin D₃. 9.26,27-dipropyl-24-dihomo-1α,25-dihydroxyvitamin D₃. 10.26,27-dipropyl-24-trihomo-1α,25-dihydroxyvitamin D₃.
 11. Apharmaceutical composition containing an effective amount of a compoundas claimed in claim 1 together with a pharmaceutically acceptableexcipient.
 12. A pharmaceutical composition as claimed in claim 11 wherethe compound is 26,27-dimethyl-24-dihomo-1α,25-dihydroxyvitamin D₃. 13.A pharmaceutical composition as claimed in claim 11 where the compoundis 26,27-dimethyl-24-trihomo-1α,25-dihydroxyvitamin D₃.
 14. Apharmaceutical composition as claimed in claim 11 where the compound is26,27-diethyl-24-dihomo-1α,25-dihydroxyvitamin D₃.
 15. A pharmaceuticalcomposition as claimed in claim 11 where the compound is26,27-diethyl-24-trihomo-1α,25-dihydroxyvitamin D₃.
 16. A pharmaceuticalcomposition as claimed in claim 11 where the compound is26,27-dipropyl-24-dihomo-1α,25-dihydroxyvitamin D₃.
 17. A pharmaceuticalcomposition as claimed in claim 11 where the compound is26,27-dipropyl-24-trihomo-1α,25-dihydroxyvitamin D₃.
 18. A method forinducing and enhancing cell differentiation in malignant cells whichcomprises exposing said cells to at least one of the compounds claimedin claim 1, in an amount sufficient to induce cell differentiation. 19.The method as claimed in claim 18 where the compound is26,27-dimethyl-24-dihomo-1α,25-dihydroxyvitamin D₃.
 20. The method asclaimed in claim 18 where the compound is26,27-dimethyl-24-trihomo-1α,25-dihydroxyvitamin D₃.
 21. The method asclaimed in claim 18, where the compound is26,27-diethyl-24-dihomo-1α,25-dihydroxyvitamin D₃.
 22. The method asclaimed in claim 18 where the compound is26,27-diethyl-24-trihomo-1α,25-dihydroxyvitamin D₃.
 23. The method asclaimed in claim 18 where the compound is26,27-dipropyl-24-dihomo-1α,25-dihydroxyvitamin D₃.
 24. The method asclaimed in claim 18 where the compound is26,27-dipropyl-24-trihomo-1α,25-dihydroxyvitamin D₃.
 25. A method fortreating neoplastic diseases which comprises administering to a subjecthaving a neoplastic disease an effective amount of at least one of thecompounds of claim
 1. 26. The method of claim 25 where the compoundadministered is 26,27-dimethyl-24-dihomo-1α,25-dihydroxyvitamin D₃. 27.The method of claim 25 where the compound administered is26,27-dimethyl-24-trihomo-1α,25-dihydroxyvitamin D₃.
 28. The method ofclaim 25 where the compound administered is26,27-diethyl-24-dihomo-1α,25-dihydroxyvitamin D₃.
 29. The method ofclaim 25 where the compound administered is26,27-diethyl-24-trihomo-1α,25dihydroxyvitamin D₃.
 30. The method ofclaim 25 where the compound administered is26,27-dipropyl-24-dihomo-1α,25-dihydroxyvitamin D₃.
 31. The method ofclaim 25 where the compound administered is26,27-dipropyl-24-trihomo-1α,25-dihydroxyvitamin D₃.